It's the all-in-one solution it provides that is the best part. There is no other external software needed. For example, email marketing, landing page generation, SEO keywords, and blogging are all part of the Hubspot package. It's a complete marketing solution.
Hubspot has great analytics and knowing what works and what doesn't in a marketing campaign by pressing a few buttons is amazing.
Integration is generally easy and reporting is outstanding.
I can see some limitations with specific 3rd party integrations although I do not have first hand experience with that. For example, integration with a third-party Professional Services Automation (PSA) tool was more difficult than anticipated. Integrations with EventBrite and GoToWebinar were also a little tricky.
Some of the export functions aren't as smooth as one would hope. A client wanted to use the API to integrate with Tableau, but we were forced to do manual source data data exports so that the data could be visualized in Tableau. This was not out-of-the-box and took some effort.
It's a great product but needs time and dedication to master. It's not a set and forget type of product. I would advise anyone new to inbound marketing to carefully research what lies ahead of them before making an investment like that. The software is just one part of the equation.